Exit capacity

[lyledunn]

I understand the general rationale in Bs9999 for estimating exit capacity of door widths. Establishing a risk profile is fairly subjective but once accepted it is then a matter of reference to a very exact number in terms on the capacity of the door. For example a pub with a risk profile of B2 requires calculation of exit capacity using the seemingly precise figure of 4.4mm per person. Can anyone tell me the derivation of this figure?

[kurnal]

We have discussed this before- heres one thread.There are many other threads if you BS9999 in the search field.

http://www.crisis-response.com/forum/index.php?topic=4600.0

[lyledunn]

Thanks Kurnal. However,  despite the interesting discussion I still cannot find the answer to my question. Was the matter resolved to your satisfaction?

[kurnal]

Lyle I think you are looking to make a science out of something that is completely arbitrary. This was never the intention of BS 9999. 

The trouble is when you start to recognise for example that a room with a high ceiling is likely to be tenable for longer than a room with a low ceiling then the first question is how much longer? Without fire modelling, and even that would be based on arbitrary judgements on the rate of fire growth etc its always going to be a best guess. But people want numbers to work with so the old post war building studies are dusted off, converted to metric equivalents, rounded up and down to suit simple conversions and arithmetic then  adjusted to two decimal places to  take account of the   comparative analysis of the potential benefits of high ceilings, early detection (perhaps) etc.

The fact that the figures are arbitrary can be proved by the fact that every one of them changed in the period between the publication of the draft and final issue of BS9999 and never any further explanation or consultation took place. At least one of the numbers in the tables is  clearly wrong due to arithmetical error if you work it through. 
But for the most part BS9999 was a good concept in providing a half way house between prescriptive solutions and fire engineering. The author applied a logical approach and gave us a means of evaluating factors not previously quantified, using figures he thought reasonable  at the time. Trouble is as the years pass the history and perspective are forgotten and  these numbers become set in stone and taken as gospel.
 

[lyledunn]

Yes Kurnal, I guess that I am. I would say that is how most non-experts would perceive things. One would expect that required exit width has a sound base in engineering principle and ratified by extensive trials.
I reported on this forum some time ago, one of our member venues was put to considerable expense by a council officer who insisted a 750mm door was increased to 800mm before the granting of an entertainment licence. I would have no doubt that said officer was acting in the belief that the minimum exits to which she was referring were something more than arbitrary!

[wee brian]

The escape stuff was all based on evacuation modelling done by Buro Happold.

I'm not sure if reports were ever produced. If they were, I never saw them.

[Tom Sutton]

As far as I understand BS 9999 was considered a halfway house between ADB and PD 7974. BS9999 states, "Guidance provided in this document gives a more transparent and flexible approach to fire safety design through use of a structured approach to risk-based design where designers can take account of varying physical and human factors. Much of the guidance in BS 9999 is based on fire safety engineering principles".

If this is the case then it is fair to assume all the calculations were produced by evacuation modelling, which is based on liquid dynamics. But Dr Keith Still has researched the subject for some time and disagrees with the concepts and has many papers that may throw light on this subject. If he is right it puts much of BS 9999 in doubt and as Kurnal has said it's all arbitrary no better than previous guidance.

http://www.open.edu/openlearn/science-maths-technology/mathematics-and-statistics/mathematics/do-crowds-behave-fluids and
http://www.gkstill.com/CV/PhD/Papers.html

[Fishy]

I'm really not sure I can agree with the view that BS 9999 is 'arbitrary' in this respect.  If (as Wee Brian says) the recommendations were based upon modelling and/or other forms of Fire Engineering analyses done by Buro Happold then that is not arbitrary - they are based upon scientific reason and ought to be treated as such.  Whether one agrees with them or not would depend entirely upon having access to the evidence.  I guess we just have to trust that FSH/14 did a good job (and why would they not have)?

Evacuation modelling is a subject in itself & my experience is that many people have their 'pet' model & they all claim to be the unique 'right' approach!  Exodus, STEPS, FDS+Evac or whatever all have a scientific basis.  For my part I don't really care how they work, so long as they have evidence that they've been validated for the particular set of circumstances they're being used to model (i.e. they're not just a PPG - Pretty Picture Generator...)!

[kurnal]

If indeed such fire engineering analyses were carried out to inform BS9999 then I am happy to withdraw my comment that the numbers are arbitrary. Clearly I have been working under a mis perception. I would add that  despite being involved and active in the consultation process this is the first I have heard of the fire engineering behind the figures and would be very interested to see details of the parameters or methodology  used. Especially in respect of staircase widths and the exit capacity of doors narrower than 1050mm!

[wee brian]

Evacuation modelling has moved on since it was just a rehash of a fluid dynamics

[Tom Sutton]

Fishy I do not accept it is based totally upon scientific principles, if it was, then every time you repeated an evacuation scenario you would come up with the same result, with fire safety engineering it depends on who does the work, what program is used and the data used, which includes programmer's reasoning and finally is fluid dynamics the correct concept.

Take one situation, when calculating ASET you need to decide the time from the Fire Alarm warning to the time of response, I am not aware of any data, so you would have to use an educated guess. I watched a TV documentary many years ago and Portsmouth Polytechnic College Psychology Department did some work for DPMO on Human Behaviour In Fires and one experiment showed the subjects waited at least 5 minutes before they reacted to the fire alarm, would you consider that a suitable time to use.

I still think the results are arbitrary, better than pure guesswork, but still arbitrary.

[Dinnertime Dave]

Take one situation, when calculating ASET you need to decide the time from the Fire Alarm warning to the time of response, I am not aware of any data, so you would have to use an educated guess.

Doesn't PD 7974-6 cover pre movement times for different occupancies.

[lyledunn]

Tom, maybe arbitrary is an unreasonable description. Perhaps "estimate" might be better. As an electrical contractor we often had to make estimates in tendering for work. We used estimation software for some of our larger jobs. That software was based on detailed time and motion studies, or so it claimed.  So a right angle bend pulled on a piece of 20mm steel conduit took 3.25 mins to mark and make. It followed that if you had ten similar bends then it would take 32.5 mins. But that was never the case, it was often miles off the mark, there were just too many variables. To allow for those variables we would factor in an additional 20% over both time and materials before adding profit. Although somewhat arbitrary, that was based on experience of similar jobs. Sometimes even that was not enough! Any way, it was always just an "estimate".
I think that I get the drift with fire safety. You can use scientific principles to arrive at definite conclusions but the variables are often so capricious that the definite conclusions need to be modified such that they end up being estimates. If we use experience along with the science then the estimates will not be so arbitrary!

[Tom Sutton]

I will live with that lylydunn I just think we need to be careful and not accept it as gospel because a computer says so, always remember garbage in garbage out.

[colin todd]

Brian, the underlying theory that you seek was in an internal committee draft, but it has been said publicly that if the public knew the theory they would be able to justify even more outlandish figures than those contained in the published version, so they were dropped from the published version, apparently on the basis that the committee members did not want the public to know stuff that would not be good for it (a bit like the Americans not releasing papers on the assassination of Kennedy.  It has also been said that it cannot now be reconstituted to let people know the truth.